Flame spraying involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface and bond thereto. A conventional flame spray gun is used for the purpose of both heating and propelling the particles. In one type of flame spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., below 100 mesh U.S. standard screen size to about 5 microns.
A flame spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. It is recognized by those of skill in the art, however, that other heating means may be used as well, such as electric arcs, resistant heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion flame spray gun, the carrier gas for the powder can be one of the combustion gases, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas. The carrier gas is generally the same as the primary plasma gas, although other gases, such as hydrocarbons, may be used in certain situations.
The nature of the coating obtained by flame spraying a metal powder can be controlled by proper selection of the composition of the powder, control of the physical nature of the powder and the use of select flame spraying conditions. It is well known and common practice to flame spray a simple mixture of ceramic powder and metal powder. It is also well known to clad ceramic powder with certain metals, particularly nickel and cobalt, for example, as taught in U.S. Pat. No. 3,254,970. Hard coatings that are quite useful may be produced with such mixtures or clad powders. Such coatings usually contain both ceramic and metal of the powder mixture that is flame sprayed.
In the manufacture of gas turbines, abradable metal compositions have been available for flame spraying onto the gas turbine parts for the purpose of reducing the clearance between the fan or compression blades and the housing. The blades seat themselves within the housing by abrading the coating.
Examples of metal-containing compositions for such abradable use are described in U.S. Pat. Nos. 3,084,064, 3,655,425 and 3,723,165. Such metal-containing compositions, however, are limited to the lower temperature portions of turbine engines, i.e., to portions below about 800.degree. C., because of the oxidizing and corrosive conditions in the higher temperature portions.
Flame sprayed oxides, such as zirconia, have been tried as abradable coatings for the higher temperature sections of turbine engines, but this has been done only with limited success. When such refractory oxides are flame sprayed with sufficient heat, such as with a plasma flame spray gun, to provide a suitably bonded and coherent coating, the abradability of the coating is poor. It has also been found that the blade tips of turbines wear excessively. When an oxide is flame sprayed under conditions of lower heat, many of the particles are not sufficiently melted and are trapped in the coating, thereby reducing the deposit efficiency. The resulting coatings have also been found to be friable and not sufficiently resistant to the erosive conditions of the high velocity gases and debris found in turbine engines.
In view of the foregoing, it is a primary object of the present invention to provide a flame spray powder for producing an abradable coating which is also erosion resistant.
It is a further object of this invention to provide a flame sprayed abradable coating suitable for use in the high temperature portions of a gas turbine engine.